1. Field of the Invention
This invention relates to a semiconductor fabrication process, and more particularly, to a method for forming a thin-film resistor.
2. Description of Related Art
The resistor is one of the most common electrical components widely used in almost every electrical device. A semiconductor device, mostly an integrated circuit, including memories and logical devices normally consists of resistors and other electrical components. The resistance provided by a resistor is proportional to the length of the resistor and the reciprocal of the cross-sectional area of the resistor; both are measured in the direction of the current. That is, the resistance of a resistor fulfills the following equation: EQU R=L/A,
Wherein is the resistivity of the resistor, L and A are the length and the cross-sectional area of the resistor respectively, and wherein both L and A are measured in the direction of the current. Conventionally, doped polysilicon is used as the material of a resistor in a semiconductor fabrication process, wherein the resistance is controlled by pre-determined L and A of the doped polysilicon layer.
As the integration of a semiconductor device is increased, all components within a semiconductor integrated circuit have to provide equivalent or better electrical properties. Hence, a downsized resistor still has to provide a required resistance. However, a conventional resistor made of doped polysilicon can only provide a limited resistance within a limited space because of the property of polysilicon. Using polysilicon resistor to provide a relatively high resistance then becomes a problem in designing and fabricating a highly integrated semiconductor device.
For overcoming the foregoing problem, new materials like SiCr having a higher resistivity than what of polysilicon are applied on the fabrication of a thin-film resistor of a highly integrated semiconductor device.
A conventional method for forming a thin-film resistor is illustrated in FIGS. 1A through 1I.
Referring to FIG. 1A, an insulator 102, a SiCr layer 104 and an aluminum layer 106 of about 2000 .ANG. in thickness are formed on a substrate 100. The insulator 102 is made of borophosphosilicate glass (BPSG) for covering the substrate 100 and devices (not shown in figure) pre-formed thereon. The aluminum layer 106 is used to prevent the SiCr layer 104 from being damaged by the follow-up dry etching process.
A patterned photoresist layer 108 is formed to expose a portion of the aluminum layer 106, as shown in FIG. 1B. By performing a dry etching process, the aluminum layer 106 is patterned. Then, the patterned aluminum layer 106a is used as a mask in the follow-up patterning process to transfer the pattern onto the SiCr layer 104, as shown in FIGS. 1C and 1D.
Referring to FIG. 1E, contact holes 110 are formed in the insulator 102, and then, filled with a conducting layer 112, wherein the conducting layer 112 also covers the patterned aluminum layer 106a and the patterned SiCr layer 104a, a thin-film resistor.
Referring next to FIG. 1F, a patterned photoresist layer 114 is formed on the conducting layer 114 for defining interconnect. After performing a dry etching process to remove a portion of the conducting layer 112, as referring to FIG. 1G, a portion of unwanted remains 115 of the conducting layer 112 often resides next to the thin-film resistor 104a. The unwanted remains of the conducting layer 115 usually cause defects, such as a short circuit, that degrades the fabrication yield.
Referring to FIG. 1H, a patterned photoresist layer 116 is formed on the substrate 100 to cover the top and lateral surfaces of the interconnect 112. By using the photoresist layer 116 as a mask, the aluminum layer 106a is removed by a wet etching process. As shown in FIG. 1I, after the photoresist layer 116 is removed, the fabrication process of a thin-film resistor is then accomplished.
Normally, the resistance of a thin-film resistor needs to be further precisely determined by a laser cutter according to a measured result on the electrical property of the thin-film resistor 104a. Then, a follow-up metallization process is performed to connect the well-defined thin-film resistor to other devices.
Though a thin-film resistor of SiCr is able to provide a relatively high resistance without occupying a large space as a polysilicon resistor does, there are still drawbacks according to the conventional process of forming a thin-film resistor.
Since the space 117 between the interconnect 112 and the thin-film resistor 104a is limited according to the increased integration of a semiconductor device, the photoresist layer 116 can not either expose the entire aluminum layer 106a or fully cover the interconnect 112. In the case that the aluminum layer 106a is not fully exposed, it can not be totally removed from the top of the thin-film resistor 104a. Therefore, the remaining aluminum on the thin-film resistor 104a degrades the performance of the thin-film resistor 104a. The remaining conducting material 115 further causes defects such as a short circuit that suppresses the fabrication yield.
On the other hand, once the photoresist layer 116 can not fully cover the interconnect 112, the lateral portion of the interconnect 112 can be etched away by the follow-up etching process. Hence, a defect like an opened circuit occurs.
Even though the aluminum layer is capable of protecting the thin-film resistor from the damages caused by dry etching processes, the provided protection is limited. Normally, the thin-film resistor still get damaged by the dry etching process even in the presence of the aluminum layer if more than two dry etching processes are performed.
Furthermore the conventional method for forming a thin-film resistor contains numerous steps and requires new equipment, so it is time-consuming and not costefficient
In addition, the conventional method for forming a thin-film resistor can not be re-modified once the follow-up metallization process is done.